1. Field of the Invention
The present invention relates to a structure of an operation member mounted on an electronic device and a camera.
2. Description of the Related Art
There are many examples of products incorporating an electronic circuit which use a slide type knob as an operation unit for instructing operation conditions, etc., to a control circuit and use a slide switch which outputs different signals according to the moving position of the knob.
An overview of an operation switch provided in a camera will be explained using FIG. 8. In the camera shown in FIG. 8, an exterior cover 22 of a camera body 21 is provided with an operation lever 23 and a switch unit provided inside the camera body 21 is operated by moving the operation lever 23.
A detailed structure of the switch unit 24 will be explained using FIG. 9. A base member 24a is made of a resin material and provided with electrical insulation properties. A first terminal 24b and a second terminal 24c are provided in the base member 24a and the terminals 24b and 24c have conductors which extend beyond the base member 24a. 
A sliding armature 24e which is formed of a metallic thin plate with spring properties, is attached to a knob member 24f formed of a resin material. When the knob member 24f is operated, the sliding armature moves together with the knob member 24f and sides on the surface of the terminals 24b and 24c. In this way, signals are output from the terminals 24b and 24c to the outside (control circuit).
A knob spring 24g forms a compression spring made of a wound spring wire member and energizes the knob member 24f toward a stationary position (rightward in FIG. 9) all the time as shown in FIG. 9. A cover member 24h covers the top surface of the base member 24a and part of the knob member 24f is exposed from a hole in the center.
In the condition shown in FIG. 9, the sliding armature 24e has contact only with the first terminal 24b and there is no electrical connection between the first terminal 24b and the second terminal 24c. 
When the operator moves the operation lever 23 provided on the exterior cover 22 from a stationary position (position in FIG. 9) to an operation position in FIG. 10, an engagement lug 23a pushes the knob member 24f and moves it from the stationary position (position in FIG. 9) to the operation position (position in FIG. 10) in the direction indicated by arrow A.
In this way, the sliding armature 24e contacts the first terminal 24b and second terminal 24c and electrically connects the two terminals 24b and 24c. An electronic circuit (not shown) detects the above described electrical connection and instructs the control circuit to change operation conditions, etc.
Then, when the operator releases the operation lever 23, the knob member 24f receives the spring force of the knob spring 24g and returns from the operation position to the stationary position. And the sliding armature 24e also returns to the position repressing electrical connection between the first terminal 24b and second terminal 24c. The electronic circuit detects the condition in which there is no electrical connection and instructs the control circuit to restore the original operation condition.
On the other hand, FIG. 11 shows an example of a switch unit 25 which has two operation positions of the knob member 24f. In FIG. 11, the same members as those described above are assigned the same reference numerals. A knob spring 24i forms a compression spring made of a wound spring wire member and energizes the knob member 24f so as to return it from the second operation position to the stationary position all the time.
In the condition shown in FIG. 11, the sliding armature 24e has contact with only the first terminal 24b and does not electrically connect the terminals 24b and 24c or the terminals 24b and 24d. 
Here, when the operator moves the operation lever 23 from the stationary position (position in FIG. 11) to the operation position as shown in FIG. 12, the engagement lug 23a pushes the knob member 24f in the direction indicated by arrow A, and moves the knob member 24f from the stationary position (position in FIG. 11) to a first operation position. In this way, the sliding armature 24e electrically connects the first terminal 24b and second terminal 24c. 
Furthermore, when the operation lever 23 is moved to a second operation position as shown in FIG. 13, the engagement lug 23a pushes the knob member 24f in the direction indicated by arrow B, and moves the knob member 24f from the stationary position to the second operation position. Therefore, the sliding armature 24e electrically connects the first terminal 24b and third terminal 24d. 
As described above, when the terminals are electrically connected, an electronic circuit (not shown) detects this condition and instructs the control circuit to change the operation condition. Then, when the operator releases the operation lever 23, the knob spring 24g or knob spring 24i causes the knob member 24f to return to the stationary position in FIG. 11. In this way, the sliding armature 24e also returns to the position repressing electrical connection between the first terminal 24b and second terminal 24c or electrical connection between the first terminal 24b and third terminal 24d. The electronic circuit detects this and instructs the control circuit to restore the original operation condition.
In the above described structure of the conventional art, the role of returning the operation lever 23 to the stationary position is assumed by the knob spring 24g (24i). However, the conventional structure alone cannot reliably return the operation lever 23 to the stationary position.
In the structure shown in FIG. 9, due to a gap between the knob member 24f and engagement lug 23a or flexure in various portions, etc., a subtle backlash occurs between the operation lever 23 and exterior cover 22.
Furthermore, even when the knob spring 24g tries to return the knob member 24f to the stationary position, friction between the sliding armature 24e and terminals 24b and 24c may repress the knob member 24f from returning to the stationary position.
Especially in the structure shown in FIG. 11, the above described problem becomes noticeable. The stationary position of the knob member 24f is a point at which a spring force F1 of the knob spring 24g and a spring force F2 of the knob spring 24g are brought into balance of “F1=F2.”
If the switch unit 25 has a symmetrical structure and the knob spring 24g and knob spring 24i are identical members, the stationary position of the knob member 24f theoretically becomes the central position equidistant from both ends of the base member 24a. However, there is normally an error of approximately 10% between the forces of the knob springs 24g and 24i as the coil springs, and therefore it is difficult to make the knob springs 24g and 24i absolutely identical in reality.
Furthermore, a frictional force which is obtained by multiplying frictional coefficient μ by contact pressure N, is produced when the sliding armature 24e contacts the terminals, and the position at which the knob member 24f stops is any position within a range where (F1±10%)=(F2±10%)±μN holds, and therefore the stationary position becomes extremely unstable.
FIG. 14 shows a relationship between a moving distance (x) of the knob member 24f and the spring force (restoring force, y) in the stationary position direction which the knob member 24f receives. In FIG. 14, the horizontal axis shows the moving distance of the knob member 24f and the vertical axis shows the restoring force generated in the knob member 24f restoring it to the stationary position.
Theoretically, the spring curve is a symmetric straight line as shown by a solid line and the knob member 24f stands still at the central position (1 point). However, due to the influence of the frictional force, etc., the restoring force actually decreases as indicated by two-dot dashed lines compared to the solid line and the stationary position of the knob member 24f falls within the range indicated by X1.
Furthermore, if there is a variation in the balance between the right and left knob springs of the knob member 24f, the central position is shifted to the right or left, and therefore the stationary position of the knob member 24f is not stable. Furthermore, a greater operation force is required as the amount of movement of the operation lever 23 increases, and therefore it is difficult to operate the operation lever 23 smoothly.